1. Field of the Invention
The invention relates to the purification of gases. More particularly, it relates to a process for enhancing the recovery of the intermediate component of adsorbable feed gas mixtures.
2. Description of the Prior Art
The pressure swing adsorption (PSA) process provides a highly desirable means for separating and purifying gases, such as hydrogen, contained in a feed gas mixture thereof with impurities that are selectively adsorbed by one or more adsorbent beds in a PSA system. Adsorption occurs in such beds at a higher adsorption pressure, with the selectively adsorbable impurities thereafter being desorbed by pressure reduction to a lower desorption pressure. The beds may be purged at said lower pressure for further desorption and removal of impurities, if desired, before repressurization to the higher adsorption pressure for adsorption of impurities from additional quantities of the feed gas mixture as the processing sequence is carried out, on a cyclic basis, in each bed in the adsorption system.
The PSA process is commonly employed in multi-bed systems. The Wagner patent U.S. Pat. No. 3,430,418, discloses a PSA process and system employing at least four adsorbent beds arranged for carrying out a particular PSA processing sequence on a cyclic basis. This sequence includes higher pressure adsorption, cocurrent depressurization to intermediate pressure with release of void space gas from the discharge or product end of the bed, countercurrent depressurization to a lower desorption pressure, and repressurization to a higher adsorption pressure. Wagner discloses the passing of released void space gas from one bed directly to another bed initially at its lower desorption pressure. The pressure in the two beds is thereby equalized at an intermediate pressure. Additional void space gas can be released from the product end of the bed being cocurrently depressurized, with such void space gas being used to provide purge gas to another bed in the PSA system before such other bed is repressurized from its lower desorption pressure. After each bed has been repressurized to an intermediate pressure level by such pressure equalization, it is further repressurized from the intermediate level in part by the countercurrent addition of product effluent to the product end of the bed being repressurized.
In a further development in the art, the Fuderer et al patent U.S. Pat. No. 3,986,849, discloses the use of at least seven adsorbent beds, with the feed gas mixture being introduced to the feed end of at least two adsorbent beds, in overlapping identical processing cycles, at all stages of the PSA processing sequence. Each bed, in turn, undergoes three pressure equalization steps prior to repressurization to the higher adsorption pressure, with said steps being carried out in a particular manner achieving higher product purity.
The PSA process, as represented by the disclosures of such patents, is a highly desirable and practical commercial process for the purification of gases such as hydrogen. It has the advantage of being capable of producing very high purity product, e.g. in excess of 99.9 percent pure hydrogen product. The PSA process can be used to treat a wide range of available feedstocks, and is not limited to use with a particular hydrogen-containing or other feed gas stream. No pretreatment or post treatment steps are required as part of the process, other than such conventional impurity removal as may be desirable or required to avoid undue degradation of the adsorbent. In addition, there is very little pressure drop between the feed gas stream and the product gas so that the product gas is available at the adsorption pressure level for further use downstream of the PSA system and for repressurization of each bed to said adsorption pressure from a lower desorption level or from the intermediate pressure level achieved during one or more pressure equalization steps carried out as indicated above.
It is also known in the art that selective adsorption process, such as the PSA process referred to above, can be employed to recover the more strongly adsorbable component from a gas mixture as the desired product. For example, the PSA process can be employed to recover 99% carbon dioxide from a gas mixture containing said carbon dioxide as a more readily adsorbable component, together with other, less readily adsorbable components, such as methane, hydrogen, nitrogen and the like. The processing cycles employed for such purposes are essentially the same as those employed when the PSA process is utilized for the production of the less readily adsorbable component.
When such well-known PSA cycles are employed for the separation of gas mixtures containing a number of components, the less readily adsorbable component and/or the more readily adsorbable component, as recovered during the adsorption and/or desorption steps, respectively, may contain components having intermediate adsorbable characteristics, vis-a-vis the more readily and the less readily adsorbable components of the mixtures themselves, depending upon the processing conditions under which the PSA process is carried out. Thus, in the separation of a gas mixture containing hydrogen, argon, nitrogen and carbon monoxide, with hydrogen being the less readily adsorbable component and carbon dioxide being the more readily adsorbable component, high purity hydrogen can be recovered with the more readily adsorbable component separated therefrom comprising a mixture of argon, nitrogen and carbon monoxide. Similarly, high purity CO can be recovered as a more readily adsorbable component with the less readily adsorbable component comprising a mixture of hydrogen, argon and nitrogen. It is also possible to employ known PSA cycles so as to obtain a lighter, less readily adsorbable component comprising hydrogen and argon, and a heavier, more readily adsorbable component comprising nitrogen and carbon monoxide.
There are practical applications in the art, however, where it would be desirable to recover the intermediately adsorbable component as a separate product of desired purity rather than to have said component recovered (1) with the less readily adsorbable component, (2) with the more readily adsorbable component, or (3) as part of a waste stream also containing said less and more readily adsorbable components. As used herein, the terms "intermediately adsorbable component" "intermediate component" and "intermediate product" are used to denote a single gas constituent or more than one such constituent of a gas mixture also having a less readily adsorbable component and a more readily adsorbable component. In the illustrative gas mixture referred to above, it may thus be desired to recover argon as an intermediate component, while recovering hydrogen as a less readily adsorbable component and a mixture of CO and nitrogen as a more readily adsorbable component. In other circumstances, it may be desirable to recover nitrogen as an intermediate component, with a mixture of hydrogen and argon comprising a less readily adsorbable component and a mixture of CO and additional nitrogen comprising the more readily adsorbable component. In another variation, it may be desirable to separate and recover a mixture of argon and nitrogen as an intermediate component, apart from hydrogen as the light, less readily adsorbable component and CO as the heavy, more readily adsorbable component. It has not hertofore been feasible to make such separations and recoveries of intermediate components of gas mixtures in a manner compatible with conventional, convenient PSA processing. Upon the development of such convenient processing capability for intermediate product recovery, those skilled in the art will appreciate that a variety of practical commercial PSA operations could be advantageously carried out so as to achieve desirable intermediate product recovery.
One approach heretofore suggested for such development of intermediate product recovery capability is set forth in European Patent Specification No. 0 008 882, published Dec. 30, 1981, in the name of Shivaji Sircar. Disclosed therein is the separation of multicomponent feed gas mixtures having a primary key component, a secondary key component and a tertiary component. For this purpose, an adsorption system comprising a plurality of bed pairs, i.e., two beds in series, is provided, with each bed pair functioning in the manner of a single bed in accordance with the conventional PSA processing techniques referred to above. The feed gas mixture passes through a first bed of the pair and then through the other, with a tertiary component adsorption front established in the first bed and a secondary key component adsorption front being established in the second bed of the pair. The flow of the feed gas mixture is then interrupted, and gas flow between the beds are discontinued. The beds are then separately subjected to rinsing, product reduction, purge and partial repressurization steps prior to the re-establishing. of flow therebetween to achieve final repressurization, initiation of the flow of the feed gas mixture thereto and repetition of the processing cycle. While the disclosed process can be employed for intermediate product recovery, the complex processing steps necessarily associated with the use of pairs of adsorbent beds in multi-feed systems, together with the associated complexity of lines, valves, controls and the like, serve to limit the practical applicability of the process. There remains in the art, thereafter, the need for an improved PSA process capable of facilitating intermediate product recovery in a practical, convenient manner.
It is an object of the invention, therefore, to provide an improved PSA process capable of enabling intermediate product recovery to be achieved.
It is another object of the invention to provide a process facilitating said intermediate product recovery in a manner compatible with conventional multi-bed PSA systems.
It is a further object of the invention to provide a PSA process having advantageous flexibility in recovering desired intermediate product from multicomponent feed gas mixtures.
With these and other objects in mind, the invention is hereinafter set forth in detail, the novel features thereof being particularly pointed out in the appended claims.